In order to recover hydrocarbons, a well is drilled into the ground until a hydrocarbon reservoir is encountered. In the earlier days of oil and gas exploration, most well sites were located onshore, and the wells that were drilled were primarily vertical. As the search for larger hydrocarbon reservoirs continues, the exploration is now focusing on offshore locations and remote land sites. Further, many wells are being drilled and completed as highly deviated and horizontal wells for economical and logistical reasons.
In offshore applications, one type of installation includes use of a fixed platform wherein the legs of the platform are rigid and embedded into the sea floor. The fixed platform has been a very popular type of structure; however, as the search for reserves continues, oil and gas companies find themselves searching in offshore locations were the water depths may be as deep as 6000 feet.
Regarding land locations, the exploration, drilling and production are now taking place in remote locations that may include arctic regions, desert regions, or even the rain forest of Latin America. Regardless of the inland or offshore location of due to, the remote nature of locations and the necessary ancillary equipment and personnel that must follow, the rental rates for these rigs are very significant.
In offshore waters, traditional fixed platforms can not be placed in depths generally greater than 300 feet. Therefore, tension leg platforms, drilling ships or semi-submersible drilling vessels are being used to drill these deep water wells. Typically, this involves the drilling rig being placed on the ship or floater. A sub-sea blow out preventor stack (BOP) is then placed on the ocean floor. A riser is then connected from the sub-sea BOP to the drill floor. The borehole can then be drilled.
Once the well has been drilled and a hydrocarbon reservoir has been encounter, the well is ready to be completed. Many sub-sea wells are completed as single satellite wells producing to a nearby platform. They are a means of producing field extremities that cannot be reached by directional drilling from an existing platform and where the economics do not justify the installation of one or more additional platforms. Some multi-well templates and piping manifolds have been installed that reach beyond the satellite well concept.
While the governments have recognized the importance and the necessity of drilling and completing wells in remote locations, significant regulations exist for each phase of the drilling, completing, and producing operation. When a certain size drill string is substituted for a second size, or alternatively, for production tubing, operators require changing of the BOP ram members so that control of the wellbore is always maintained. This is a crucial concern because control of the wellbore is essential at all times.
When the operator is converting from the drilling phase to the completion phase, the BOP stack must be changed to accommodate the different outer diameter sized work string--from drill pipe to a production string. Furthermore, during the actual completion phase, the production tubing must be manipulated in order to perform the necessary functions such as perforating, circulating, gravel packing and testing. According to established safety procedures mandated by operator rules and government regulations, it is necessary to change the BOP rams during certain phases. The changing of the BOP rams can be a costly and time consuming practice. Day rates for drill ships and semi-submersible ships can be quite expensive, and during the procedure for changing the rams, no other substantive operations can be accomplished.
In a typical offshore location, wherein the drilling rig is either a jack-up vessel or placed upon a fixed platform, the BOP is normally situated on the vessel or platform itself. Nevertheless, because of safety considerations and government regulations, the control of the well bore from blow-out is always of primary concern. Therefore, safety of the installation along with economically performing the operation has always been a need.
In order to minimize cost, several techniques have been employed with varying degrees of success. One technique has been to drill and case the well, and then immobilize the drilling rig. A replacement rig is then utilized to complete the well. The replacement rig may vary from a snubbing unit, coiled tubing unit, work over rig using smaller inner diameter pipe, and in some cases wire line. Thus, rather than completing the well with the more expensive rig, a less expensive rig is utilized. Therefore, there is a need to provide for a more cost effective means for drilling and completing wells in the exotic locations of the world.